The invention relates generally to communication systems, and more particularly to a method for adaptive frequency sharing for use in a frequency hopping communication system.
Frequency hopping is a commonly used technique to provide a secured communication system. Frequency hopping involves the changing of the radio carrier or center frequencies periodically to avoid detection or jamming. That is, frequency hopping transmission systems are a type of spread spectrum system in which a wideband signal is generated by hopping from one frequency to another over a large number of frequency choices. In systems employing very fast frequency hopping, the signal is transmitted at each frequency for a very short period such as 20 milliseconds. The frequencies used are chosen by a pseudo random code similar to those used in direct sequence systems. For general background on spread spectrum systems, reference is made to a text entitled Spread Spectrum Systems, 2nd edition, by Robert C. Dixon and published by Wiley-Interscience, New York (1984). In order to increase the efficiency of digital radios employing spread spectrum characteristics, digital engineers have raised the number of modulation levels and have generally dealt with modulation/demodulation techniques, spectral shaping and synchronization schemes. This has led to widespread and more efficient use of the digital radio systems.
As one can ascertain, a major problem in the transmission of digital data signals is interference. Hence, interference or noise, in any communication system, plays an important part in the design and operation of the system. For general background on digital radio and particularly for microwave operation, reference is made to a text entitled Microwave Digital Radio edited by Larry J. Greenspan and Mansoor Shafi and published by the IEEE Press, the Institute of Electrical and Electronic Engineers, New York (1988).
Frequency Hop (FH) radios spread their transmitted RF energy over a wide frequency spectrum by transmitting only a short burst of data on any one frequency in the allocated frequency spectrum. FH radio effectiveness with jamming and interference is based on having a large hop set of frequencies spread over a wide frequency spectrum. The object is to either make a jammer or an interferer spread its energy over the full spectrum of hop set frequencies, thereby reducing the amount of interference energy on any single frequency, or to correct the errors due to interference on a small number of frequencies by the use forward error correction coding on the transmitted data. The frequencies used are chosen by a pseudo noise code similar to those used in direct sequence systems.
FH radios perform best when greater than half of the frequencies in the hop set do not contain interference. Otherwise, the required overhead associated with low rate, forward error correction code consumes a large percentage of the channel capacity and results in a large reduction in data throughput. Unfortunately, in government or military operations, allocation of frequencies is often restricted to a small set of available frequencies.
Because FH operation does not perform well where the number of hop set frequencies is restricted, the users of such FH radios experience less-than-optimal communication performance in the field. This frequency shortage has become a more severe problem with the large amount of wireless communications required for both military and civilian communications. Permission to use a large frequency hop set for military exercises and for joint NATO operations is becoming a more intense problem. In addition to the smaller frequency hop set, more FH nets are being required to share the same frequency hop set, further increasing the mutual interference.
Accordingly, a method for minimizing interference in a frequency hopping network whereby a single frequency may be selected for data communications, which is within a predetermined tolerance level and which has low interference energy to permit data transmission on that frequency, is highly desired.
It is an object of the present invention to provide in a frequency hopping (FH) communication system comprising a plurality of radios operable in a first FH mode for transceiving information over a plurality of frequency channels corresponding to frequencies defining a frequency hop set, a method for adaptively selecting a single frequency channel within said hop set and transceiving said information over only said single frequency channel, said method comprising the steps of determining the quality of frequency hopping transmissions received at a receiving radio terminal in the first FH mode and providing a control signal when said quality is unacceptable; and operating the radio in a single frequency mode in response to the control signal when the FH transmission quality is unacceptable by transmitting the information over only a single frequency channel within the frequency hopping set which has an interfering energy level below a threshold value indicative of acceptable transmission quality.